Heretofore, there is known a traction drive transmission device which uses a tangential force which acts on a contact portion of a body of revolution which is in a rolling sliding state, to perform transmission of a torque having a transmission ratio between the number of revolutions of an input shaft and the number of revolution of an output shaft. In this traction drive transmission device, the input/output transmission ratio becomes a linear shape (for example refer to Patent Document 1).
Furthermore, as a transmission device in which linear shape stepless transmission is possible, there is know a planetary roller type transmission device (for example refer to Patent Document 2).
Moreover, there is known a vehicle steering device in which a reduction gear is arranged in a torque transmission path of a steering transmission system provided with a differential transmission mechanism which functions as a step-up gear, to thereby step-down the transmission ratio which has been stepped up, and finally return the transmission ratio of the overall steering system to, for example, 1:1 (for example refer to Patent Document 3).    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2003-278866    Patent Document 2: Japanese Unexamined Patent Application, Publication No. Hei 1-139161    Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2004-58896